what is the evidence for geomagnetic reversal ?

rogerharris

Apologies if this has been raised. Could not find what i was looking for.

We are being given paleomagnetics as part of a basic science degree, and i was having problems getting what the evidence is for geomagnetic reversal. My tutor could not answer this question and tells me to move on to the current assignments. I keep coming back to this. Damn curiosity !

the idea for the reversal theory is that magnetite aggregations have Subchronozone distributions clearly reversing their domain orientations across the planet within the same time frame.

However when i dig out papers i only find one section of the Atlantic where this is being explored and some mountain areas where domains went the opposite direction. at the same time we are being told that in these active mountain building areas the rock recycling could flip a crustal section 180 degrees anyway.

Ok my point is this, i originally thought that the magnetic bands on earth wide sea floor spreading were marked black and white because the magnetite domain ordering had flipped 180 degrees. i.e. north becomes south and vice versa. And that was our earth wide time synchronized evidence. However it seems that the bands are really areas where the magnetic field has become weak or the domains shift by slight degrees not 180 degrees ! Is this correct ? If so this just tells us that there has been earth wide pole wander and not reversal.

Am i missing the obvious or did i misinterpret what i read ?

what is the evidence that clearly distinguishes regular earth-wide pole wander from actual regular 180 degree reversal ?

Evo

This post explains both pole shifts and pole reversals (magnetic field).

http://www.physicsforums.com/showpos...85&postcount=9

The entire thread http://www.physicsforums.com/showthread.php?t=406653

Integral

The evidence lies in the Mid Atlantic ridge. The Mid Atlantic Ridge is formed by an up welling of new rock. The magnetic field of the upwelling material is fixed when the molten rock solidifies. Thus as the ridge grows it leaves a time map of the magnetic field. It is in these rocks that they find periodic reversals of the magnetic field.

Evo

They find evidence many places.

http://www3.geosc.psu.edu/~jte2/geosc20/lect26.html

Andre

Much of the sediment cores drilled in the ocean in the Ocean Drilling Program are magnetized very weakly, yet detectable in labs like this.

Since the magnetic polarization proves to be very consistent in time/depth, it's one of the more popular methods to date the cores back hundreds of million years, using the geomagnetic pole reversals.

Here is an example of correlating geomagnetic dating of a lot of oceanic sediment cores (fig 4 on page 6):



Notice the black/white bar above the time scales with the names like Brunhes, Matuyama, jaramillo, Olduvai, etc, those are the geomagnetic "chrons", between geomagnetic pole reversals, which are dated very accurately, over and over again.

geo101

As folk have pointed out above there is a range of evidence from different sources that support geomagnetic reversals. I think some of your confusion lies in the mechanisms by which rocks can record the geomagnetic field. In particular the confusion between reversals of the geomagnetic field and the phenomenon of self-reversal behaviour, which is seen in some rocks.

I don’t have time to go into a detailed description right now, but, if your library has it, I would recommend Paleomagnetism: Continents and Oceans (2000) by McElhinny and McFadden (ISBN 0-12-483355-1). Chapter 4 explains the background, self-reversals and the evidence supporting geomagnetic field reversals.

And by the sounds of it, your tutor should read it too. It is an excellent all round paleomag book.

rogerharris

thanks, thats the kind of in depth stuff i was trying to find.

i will need to take some time out to get through that, as i cant see from this data where the field reverses rather than weakens, but i presume it just needs more reading and what this papers does is this.

1. plots distributions of the direction of the earths magnetic domains at each period in time.

2. Finds these are consistent with a single direction across the entire planet at a resolution that is reasonable to provide an overall picture and that this overal picture reguarly flips orientation domains.

3. seeks to falsify the data by seeking out local heat related domain flips as well as crustal overturn which both which can confound the data.

rogerharris

annoying. google books has all but chapter 4 and the uni library only has reviews. However i recognize the name McElhinny and think they put out a seminal paper which might provide a brief overview.

I do recall somebody pointing out that McElhinny can not so confidently extract latitude of the orientation of magnetic domains from the data as has been carried out. finals on other stuff so a come back later for project.

rogerharris

thanks for other replies. Primarily trying extract the in depth technical data for this subject. Not easy, there appears to be more geology books than journal publications for some reason. i guess thats due to them being out on the field most of the time.

billiards

The surveying of the earth's magnetic field over the oceans provided the most compelling evidence for field reversals in conjunction with evidence for sea floor spreading. It is important to note that the anomalies are formed into the newly formed rock as it is cooled below its Curie temperature at the mid-ocean ridge. However, these anomalies migrate away from the ridge as the ocean floor physically moves away from the spreading centre. The point being that the evidence is not only found at the ridge; the evidence is found over the entire ocean floor. The compelling thing about it is that the same pattern of magnetic reversals is found either side of the spreading ridge. It really is beautiful.



True. But the link you have provided does not help us to find out where those other places are.

I wouldn't spend too much time worrying about this. The plots are not evidence for magnetic reversals. They are using the magnetic reversals as a dating mechanism for a geochemical property of the rocks. This does not get at the heart of the question you originally asked.

billiards

The issue is that when we are making a measurement of the magnetic field we are measuring everything. Whatever sources of magnetic field are out there, be they the Earth's magnetic field today or the rocks of 50 million years ago, we are measuring it all together in one bulk measurement (not to mention all the sources of "noise").

Now we can isolate bits of rock and analyse the orientation of their magnetic field in the lab. But that is not what was routinely done by the US military when they first started surveying the oceans. They just measured the whole field, and what they found was that the field got a bit stronger here, then a bit weaker over there, and then a bit stronger again, and when they plotted it all up it looked kind of stripy and they thought that that was kind of interesting. Why would the magnetic field look like that?

It turns out that it is because of the alignment of "magnetic domains" in the rocks. When the rocks were created they were very hot, they were above their Curie temperature and as the rock cooled the crystal in the rock formed little magnetic domains which lined up with the prevailing geomagnetic field and were frozen solid in place. Then the field reversed, and the new rocks that were being created all lined up their magnetic domains in the opposite direction.

So now when we measure the magnetic field we find that the magnetic field is stronger if the rocks were formed at a time when the magnetic field was in the same direction as it is today -- because it adds to the Earth's magnetic field. When we run our magnetometer over some rocks that were formed at a time when the magnetic field was in the opposite direction to what it is today we find that the magnetic field is weaker, because the magnetic field of the rocks is "fighting" against the earth's present magnetic field. So what we measure is a strengthening and weakening of the magnetic field today, but this is caused by complete field reversals in the past. To confirm this you would need to look closely at samples of the rock in the lab and to measure the direction of the magnetic field -- these studies have been done.

OmCheeto

Doesn't one of the theories regarding field reversal say that the main field will start to wander and break up?

I was just looking at the following field map:


Figure 2. Total magnetic field presented in nanoTeslas
(please click on the image for the reference.)

And it looks as though we currently have two magnetic north poles, and one magnetic south pole.

Is this correct?

disclaimer: I am not a geophysicist, have never been one, and doubt I ever will be. But I do like playing with magnets.

Evo

What the earth's magnetic fields are supposed to look like during a reversal.

http://en.wikipedia.org/wiki/Geomagn...of_transitions

A good article from NASA.

http://www.nasa.gov/vision/earth/loo...eticfield.html

Attached Thumbnails

 

rogerharris

Ok thanks, well explained. so basically where the field is weaker the magnetic domains are actually completely flipped but this is hard to tell in the ground. so in the ground we really cant know if its flipped or just wandered.

A piece of this weak rock has to be cut out, with its north pole labeled, and another piece cut out where the field is strong and similarly labeled, there would be complete dipole reversals of each other under lab conditions with magnetic shielding from the earths background field(presuming the magnetic oxides are similar) ?

If anybody knows the best papers which summarize that evidence where its completely clear that we have flipping and not just wander or fluctuation that would be useful thanks.

rogerharris

yeh i have seen these simulations, but had the problem they may have modeled presumptions.

another aspect i don't get is that the hard ferromagnetic core where the bulk of the magnetic field comes from, how does this undergo a domain flip due to paramagnetic fluids sloshing around it. These fluids are kind of random in their turbulence and they put out a weaker field than the core, so where do they get the consistency and magnetic power to re-orientate the hardcores magnetic dipole.

Especially as the hard core itself remains cooler due to convection so its curie temp is not being raised. So there is another question, what is the actual reversal mechanism. These new models point more to the idea that the soft outer core can really just modulate and not flip the hard cores dipole.


these are the recent articles on the hard core of the earth being a lower temperature dipole BTW. It even says "A cross-section of the earth's interior shows the outer crust, the hot gooey mantle, the liquid outer core and the solid, frozen inner core (gray). (Calvin J. Hamilton graphic)"

Although i dont know what they mean by frozen as in it was frozen and its temp is higher now.

report from Berkeley uni on the findings

letter of it submitted to science mag

the technicalities in the publication to nature

it says in the news report

"The Earth’s magnetic field is produced in the outer two-thirds of the planet’s iron/nickel core. This outer core, about 1,400 miles thick, is liquid, while the inner core is a frozen iron and nickel wrecking ball with a radius of about 800 miles – roughly the size of the moon. The core is surrounded by a hot, gooey mantle and a rigid surface crust."

So if the inner core is 800 miles radius of compressed and cool iron/ nickel would this not be a pretty strong magnetic dipole structure on its own.

Evo

Have you read any of the papers suggested at the bottom of the NASA article?

Like this one?

http://es.ucsc.edu/~glatz/geodynamo.html

billiards

So the theory is that whatever field was prevalent at the time the rock cooled below its Curie temperature will be locked into the rock. Actually you'll find that it can alter a bit with weathering, and there is alway the distinct possibility that the rock may be tilted -- even completely overturned -- by later geotectonic activity. It's never quite THAT simple.

But yes, if you wish to find the magnetic field of the rock (and then infer that this represents the geomagnetic field at the time of its formation) then you need to isolate a sample of it.

This is not my area and so I do not know the BEST papers. You could try this one (http://www.springerlink.com/content/c5m8212702058nw8/) to see a study of the rocks in the Deccan Traps -- which themselves are extremely interesting for other geological reasons.